Method for preparing diamond from graphite by inner shell electron excitation

ABSTRACT

If a stable graphite structure is reversed to a stable diamond structure using some excited state established at normal temperature and pressure, graphite can easily be transformed into diamond. If this technique is realized, it is expected to be applied to nanotechnology, short-wavelength, high-power semiconductor lasers, and high-power electronics. A method for producing diamond includes the step of exposing single-crystal or polycrystalline graphite having an sp 2  structure to one selected from the group consisting of synchrotron radiation X-rays, radiation, laser light, an electron beam, and accelerated multicharged ions under normal pressure to excite the is inner-shell electrons of carbon atoms (C) constituting the graphite, thereby producing diamond having an sp 3  structure from the graphite having the sp 2  structure. The method inexpensively produces single-crystal diamond, polycrystalline diamond, or nanostructural diamond in a large amount.

TECHNICAL FIELD

The present invention relates to a method for producing single-crystaldiamond, polycrystalline diamond, and nanostructural diamond constitutedof nanoscale particles, by inner-shell electron excitation usinggraphite as a starting material.

BACKGROUND ART

For synthesizing diamond, in general, graphite may be transformed into adiamond structure in the presence of a nickel catalyst under highpressure, or hydrogenated carbon gas may be photolyzed to be deposited.These methods, however, produce only polycrystalline or microcrystallinediamond. The known methods involve significant disadvantages that theresulting crystals have poor quality and contain a large amount ofimpurities, and thus do not lead to the production of low-resistancen-type single crystals. Therefore, there is no application to the fieldof semiconductor devices.

Also, in the existing diamond production, not only single-crystaldiamond but also nanostructural diamond constituted of nanoscaleparticles and doped nanostructural diamond used as functionalsemiconductor are not produced in controlled size with high precision.

Graphite is more thermodynamically stable than diamond, in a thermalequilibrium state under conditions of normal temperature and pressure.Diamond is used in a variety of applications, such as high-temperature,high-power semiconductor devices, UV lasers, and functionalsemiconductors including semiconductor spin electronics. In general,diamond is synthesized from a starting material graphite in the presenceof a catalyst under conditions of high temperature and pressure becauseit is metastable at normal temperature and pressure.

The inventors of the present invention have devised an alternative tothe above-described known methods, for synthesizing single crystaldiamond with hydrogenated amorphous carbon (Patent Document 1). PatentDocuments 2 and 3 have disclosed methods for producing diamond thinfilms and the like by exposing a carbon material containing —C≡C—Cand/or =c=to at least one selected from among light, an electron beam,and an ion beam.

In addition, if a stable graphite structure is reversed to a stablediamond structure using some excited state established at normaltemperature and pressure, graphite can easily be transformed intodiamond. If this technique is realized, it is expected to be applied tonanotechnology, short-wavelength, high-power semiconductor lasers, andhigh-power electronics.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 9-20593 (Japanese Patent No. 3232470)-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 11-310408-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2000-16806

DISCLOSURE OF INVENTION

In order to achieve the above-described challenge, the inventors of thepresent invention have developed a novel method for producing diamond bynon-equilibrium inner-shell excitation. The method producessingle-crystal diamond, polycrystalline diamond, and nanostructuraldiamond from graphite by is inner-shell electron excitation of carbonatoms (C).

Since the known method of diamond synthesis is performed under highpressure, contamination from air or the catalyst often occurs, and theproduct is a low crystalline aggregate of polycrystal or microcrystal.However, the method of the present invention can structurally transforma graphite thin film into a single-crystal diamond thin film,maintaining the original form, and also structurally transform graphitedeposited on a semiconductor substrate into diamond, maintaining theoriginal form.

In the present invention, single crystal or polycrystalline graphite isexposed to high-energy light, electrons, or ions such as synchrotronradiation X-rays, laser light, radiation, an electron beam, oraccelerated multicharged ions to excite inner-shell is electrons of thecarbon atoms (c) constituting the graphite, and thus to introduceinner-shell is holes, thereby making the excited carbon atoms stable ina diamond structure rather than in a graphite structure. Thus, a largeamount of diamond can be inexpensively produced from graphite.

Specifically, the present invention is directed to a method in which ansp³ diamond structure is produced from the sp² structure of graphite byexposing single-crystal or polycrystalline graphite to any one ofsynchrotron radiation X-rays, radiation, laser light, an electron beam,and accelerated multicharged ions to excite the is inner-shell electronsof sp²-bonded carbon atoms (C) constituting the graphite under normalpressure.

In the method, exposure time may be controlled so as to producesingle-crystal diamond, polycrystalline diamond, or nanostructuraldiamond constituted of nanoscale particles according to the controlledexposure time.

In the method, exposure time, exposure intensity, or temperature of asubstrate on which the graphite lies may be controlled so as to producemicrocrystalline diamond or nanostructural diamond according to thecontrolled conditions.

In the method, the graphite used as a starting material may contain atleast one type of dopant atoms selected from the group consisting oftransition metal dopant, rare-earth metal dopant, donor dopant, andacceptor dopant and thus the diamond contains the dopant atoms.

In the method, the graphite having the sp² structure may be providedwith a gate, a source, and a drain with a metal to prepare afield-effect transistor (FET), and a negative gate voltage is applied tothe FET to dope the crystal of the graphite with a hole, whereby thediamond having the sp³ structure becomes more stable than the graphitehaving the sp² structure.

In the method, the graphite may be doped with an acceptor, such as Batoms, Al atoms, Ga atoms, or In atoms, to allow the diamond of sp³structure to be more stable than the graphite of sp² structure.

(Operation)

FIG. 1 is a schematic diagram showing the excitation of an inner-shell1s electron of a carbon atom constituting graphite. In the method of thepresent invention, the reaction of structurally changing graphite intodiamond occurs in a solid phase.

When an electron in the is core level (A) of an carbon atom constitutinggraphite having an sp² structure is excited by high-energy light,electrons, or ions such as synchrotron radiation X-rays, laser light,radiation, an electron beam, or accelerated multicharged ions, aninner-shell 1s electron (C) of the carbon atom is emitted from thegraphite to a vacuum through the surface of the graphite, and thus ainner-shell excitation state having an inner-shell hole (D) is formedfor 1 to 20 fs. Although the inner-shell hole is blocked with electronsin a valence band (E) and conducting band (F), it has a lifetime of 1 to20 fs. The energy (G) level in FIG. 1 increases upward, as designated bythe arrow.

FIG. 2 is a graph showing, as functions of interatomic distance, thetotal energies of a diamond structure and a graphite structure (R:graphite interlayer distance, θ: graphite interatomic angle) in whichinner-shell is electrons are excited to have a hole in each isinner-shell orbital.

Even in such an excited state, the graphite structure is moreenergetically stable than the diamond structure. As designated by thedotted line along arrow A in FIG. 2, the energy state of the graphitestructure is almost restored from the inner shell excited state to aninitial state. However, since the transformation from the stablegraphite structure to a metastable diamond structure involves an excitedstate, there is some limited probability of transforming in thedirection of the solid line designated by arrow B though the probabilityis much lower than that in the ground state.

Unfortunately, the lifetime of the excited inner-shell is electron is asshort as 1 to 20 fs, and accordingly carbon atoms can move only a shortdistance in such a short period. Hence, the probability that the stablegraphite transforms into metastable diamond is low.

FIG. 3 is a schematic diagram of a two-holes-constrained state (H) inwhich two holes are constrained in a valence band, (E), and which isformed by transfer from the excited state of the inner-shell is electronof the carbon constituting graphite and by subsequent Auger effect(decay).

As shown in FIG. 3, in 1 to 20 fs after an electron of a carbon in theis core level (A) is excited, Auger effect (decay) occurs and a 2pvalence electron falls from the valence band (E) to the 1s inner-shellhole due to dipole transition. At the same time, in order to conservethe energy in the atomic position, a valence electron (C) having thesame energy but a reverse momentum is emitted into a vacuum according tothe low of conservation of energy and the low of conservation ofmomentum. Thus, two holes are formed in the valence band (E).Consequently, a two-holes-constrained state (H) is formed in which thetwo holes are localized in the valence band (E).

Thus, the carbons in the graphite structure can be transformed to afinal ionic state, C²⁺. Since the lifetime of the final ionic state C²⁺is as long as one picosecond to several tens of picoseconds, light Catoms can sufficiently widely move due to lattice vibration in such along time.

In the C²⁺ final state (two-holes-constrained excitation state), the πband constituted of a pz orbital in the valence band lacks an electron.By filling this band with electrons, the sp² structure of graphite isstabilized, and thus graphite in a layered structure is stabilized.Therefore, the two-holes-constrained state, in which the band lacks intwo electrons for its final state due to inner-shell excitation andAuger effect, allows diamond having an sp³ structure to be more stablethan graphite having an sp² structure.

FIG. 4 is a graph showing, as functions of interatomic distance, thetotal energies of a graphite structure and a diamond structure being thefinal state for the two-holes-constrained sate resulting from Augereffect. The structures are transformed along the functional curves, andthe graphite structure becomes unstable in such excited state, thustransforming into a diamond structure spontaneously.

In the final state (excited state) having two constrained holes, or C²⁺,the total energy of diamond having an sp³ structure is lower than thatof graphite having a sp² structure, in contrast to the ground state.Therefore, a graphite structure can be spontaneously transformed intodiamond by lattice vibration in an excited state.

By using the reversed phenomena of the stable structure in the excitedstate, which is completely different from the ground state, thecrystalline structure of graphite can be gradually transformed into thatof diamond, and thus the diamond structure can be stabilized.

The diamond structure depends on exposure time (excitation length) of,for example, synchrotron radiation X-ray. Specifically, single-crystaldiamond is obtained by setting the exposure time on the order ofminutes, or in the range of about 1 to 5 minutes; nanostructuraldiamond, on the order of seconds, or in the range of about 1 to 5seconds; polycrystalline diamond, between these periods of time.

FIG. 5 is a schematic illustration of an apparatus for carrying out themethod of the present invention. As shown in FIG. 5, single-crystal orpolycrystalline graphite 2 is put on a diamond or sapphire substrate 1.The temperature of the substrate is controlled with a heater 3, or acooling liquid circulator 4 and a substrate cooler 5 to maintain thegraphite at a predetermined temperature. The atmosphere is notnecessarily a vacuum, but normal atmosphere oxidizes diamond to consumeit. Therefore, the method is generally performed in a vacuum or a raregas atmosphere.

Then, any one of synchrotron radiation X-rays, radiation, laser light,an electron beam, and accelerated multicharged ions is emitted onto thesurface of the graphite 2 to excite the 1s inner-shell electrons of thelayered carbon atoms (C) in the graphite. For example, synchrotronradiation X-rays 6 having an energy between 500 eV and 2,000 eV isemitted onto the surface of the graphite 2 under normal pressure. Theenergy of the emitted light must be at least 1s core level. By emittingintense soft X-rays from the synchrotron radiation, the region of thegraphite exposed to the synchrotron radiation X-rays is entirelytransformed into a diamond structure at normal temperature and pressure.

By setting the substrate temperature, which is a parameter forcontrolling the strength of atomic diffusion, in the range of 600 to900° C., perfect single crystal can be produced. By setting thetemperature of the substrate on which the graphite lies at a temperatureof at least 900° C., and by emitting X-rays from the synchrotronradiation for 1 to 5 minutes, the single-crystal graphite can besubstantially completely transformed into single-crystal diamond by 1sinner-shell electron excitation. If the temperature of the substrate onwhich the graphite lies is reduced to a level lower than a temperatureof liquid nitrogen at which atoms are not diffused by heat, but byelectron excitation, the graphite turns into a polycrystal.

By varying the intensity of the exposure, such synchrotron radiationX-rays, exposure time, and the temperature of the substrate on which thegraphite lies, microcrystalline diamond having a small size ornanostructural diamond having a size between several tens of nanometersand several hundred nanometers can also be produced.

An acceptor (B, Al, or In) or a donor (P or N) may be added to thestarting material single-crystal graphite as a dopant, or a transitionmetal, such as Cr or Mn, or a rare earth metal, such as Gd may bedeposited on the graphite as a dopant to be compounded. Thus, by addingsuch a dopant to the nanostructural diamond and by exciting theinner-shell electron, p-type or n-type diamond or functionalnanostructural diamond having ferromagnetism or a spin glass state canbe produced.

Graphite having an sp² structure may be provided with a gate, a source,and a drain with a metal to prepare a field-effect transistor (FET), anda negative gate voltage is applied to the FET to dope the crystal with ahole. Thus, the sp³ structural diamond becomes more stable than the sp²structural graphite, and accordingly a sable diamond structure results.

Also, by heavily doping the carbon atoms (C) of sp² structural graphitewith an acceptor, such as B atoms, Al atoms, Ga atoms, or In atoms, sp³structural diamond can become more stable than the sp² structuregraphite.

By setting the temperature of the substrate on which a single-crystalgraphite lies at a liquid nitrogen temperature or less, nanostructuraldiamond including a dopant deposited on the graphite can be produced byinner-shell electron excitation. The size can be varied from 1.5nanometers to 6 nanometers. Small crystals exhibit luminescence 10 to120 times as strong as that of a bulk crystal due to a nanoscale excitonconfinement effect. These products promise to be used in nanostructurelasers.

For exposure to UV laser light, inner-shell electrons are excited bymultiphoton excitation because UV laser light is generally has a lowenergy. Also, strong laser light excites valence electrons to form astate doped with holes, thereby producing diamond from graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the excitation of an inner-shellis electron of a carbon atom (C) constituting graphite.

FIG. 2 is a graph of comparison in total energy as functions of carboninteratomic distance (R: interatomic distance; c: lattice constant)between diamond structure and graphite structure in carbon (C)inner-shell is electron excitation.

FIG. 3 is a schematic diagram of a final state in which two holes areconstrained in a valence band, and which is formed by Auger effect(decay) from the excited state of the inner-shell is electron of acarbon atom (C) constituting graphite.

FIG. 4 is a graph of comparison in total energy as functions of carboninteratomic distance (R: interatomic distance; c: lattice constant)between diamond structure and graphite structure in a final state wheretwo holes are constrained in a valence band, formed by carbon (C)inner-shell is electron excitation and subsequent Auger effect (decay).

FIG. 5 is a schematic illustration of an apparatus for carrying out themethod of the present invention.

FIG. 6(a) is a graph of the coordination distribution by X-raydiffraction analysis of graphite before exposure to synchrotronradiation X-rays (three sp² coordinations represent 100% of thedistribution); FIG. 6(b) is a graph of the coordination distribution byX-ray diffraction analysis of graphite after exposure to synchrotronradiation X-rays (four sp³ coordinations represent 100% of thedistribution). FIG. 7 shows graphs for comparison in intensity of Ramanscattering spectrometry between (a) graphite of sp² three-coordinationstructure at 2800 cm⁻¹ before exposure to synchrotron radiation X-raysand (b) diamond of sp³ four-coordination structure at 3300 cm⁻¹ afterexposure to synchrotron radiation X-ray.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES Example 1

Example of Single-Crystal Diamond Production

As shown in FIG. 5, layered single-crystal graphite lying on a sapphiresubstrate, maintained at 300° C. was exposed to synchrotron radiationX-rays having an energy of 500 to 2,000 eV for one minute under normalpressure to excite the carbon (C) 1s inner-shell electron.

The excitation of the inner shell electron from the 1s core level to avacuum was confirmed by inner-shell electron spectrometry. A final statein which two holes were constrained in the carbon 2p orbital wasobserved by Auger spectrometry. As a result, it was confirmed by X-raydiffraction analysis and Raman scattering spectrometry that the layeredsingle-crystal graphite which had been put on the substrate wastransformed into single-crystal diamond.

The coordination distribution of the graphite before exposure tosynchrotron radiation X-rays was analyzed by X-ray diffraction. As aresult, it has been found that three sp² coordinations representsubstantially 100% (FIG. 6 a).

Also, the coordination distribution of the resulting diamond afterexposure to synchrotron radiation X-rays was analyzed. As a result, ithas been found that four sp³ coordinations represent substantially 100%.Thus, the graphite was substantially completely transformed into diamond(FIG. 6 b).

Furthermore, the Raman scattering spectrum intensity (FIG. 7 a) at 2800cm⁻¹ was examined for the sp² three-coordination structure of graphitebefore exposure to synchrotron radiation X-rays. The peak was completelyshifted to a peak at 3300 cm⁻¹ of sp³ four-coordination structure (FIG.7 b). It has therefore been found that the graphite was substantiallycompletely transformed into diamond by inner-shell excitation.

Example 2

Example of nanostructural diamond production As shown in FIG. 5, layeredsingle-crystal graphite lying on a sapphire substrate was exposed tosynchrotron radiation X-rays having an energy of 500 to 2,000 eV forfive minutes at room temperature under normal pressure to excite thecarbon (C) 1s inner-shell electron.

The excitation of the inner shell electron from the 1s core level to avacuum was confirmed by inner-shell electron spectrometry. A final statein which two holes were constrained in the carbon 2p orbital wasobserved by Auger spectrometry.

As a result, it was confirmed by X-ray diffraction analysis and Ramanscattering that the layered single crystal graphite was transformed intonanostructural diamond crystals by appropriately setting the intensityof the synchrotron radiation X-rays and the temperature of the substrateon which the graphite was put.

Example 3

By doping graphite being the starting material in advance with atransition metal dopant or rare-earth metal dopant, B atoms as acceptor,and Be atoms as donor, diamond can be produced at a lower pressure thanin the case where graphite is singly used, and the resulting diamond hasferromagnetism or a spin glass state. Holes acting as carriers are dopedwith the implanted acceptor, and thereby ferromagnetic exchangeinteraction acts on the rare-earth metal or transition metal toestablish ferromagnetism. While a system doped with Cr or Mn turns intoan antiferromagnetic spin glass state, a system doped with Fe turns intoferromagnetic. In production of diamond from undoped graphite under highpressure, transition from graphite to diamond was not observed untiltemperature and pressure come to high values (1500° C., 50 GPa). Incontrast, a starting material graphite doped with 6% to 10% of atransition metal or rare-earth metal and the above-described acceptorselected from among B atoms, Al atoms, Ga atoms, and In atoms, wastransformed into diamond at a low temperature (800° C.) and a lowpressure (15 GPa). Conditions (temperature, pressure) under whichgraphite was transformed into diamond was as follows:

-   (1) Graphite alone: high temperature (1500° C.) and high pressure    (52 GPa)-   (2) Graphite+acceptor Be atoms (6%), Mn atoms (6%): high temperature    (1300° C.) and high pressure (45 GPa): spin glass state-   (3) Graphite+acceptor Al atoms (8%), Fe atoms (8%): high temperature    (1100° C.) and high pressure (28 GPa): ferromagnetic state-   (4) Graphite+acceptor Ga atoms (9%), Mn atoms (6%): high temperature    (1100° C.) and high pressure (25 GPa): ferromagnetic state-   (5) Graphite+acceptor In atoms (10%), Mn atoms (6%): high    temperature (900° C.) and high pressure (15 GPa): ferromagnetic    state

Example 4

A SiO₂ insulating film is deposited on the surfaces of the carbon atomsconstituting graphite, and aluminium is also deposited on the SiO₂ filmto form a gate. Then, a negative voltage is applied to the gate todeposit a large amount of microcrystalline diamond on the surface of thegraphite. This suggests that the graphite is doped with a large amountof holes by the application of gate voltage, particularly application ofnegative voltage, so that the energy of diamond structure becomes lowerthan that of graphite structure and, thus, the diamond structure is morestabilized spontaneously. Also, it has been found that in production ofdiamond by applying high pressure, graphite can be transformed intodiamond at lower temperature and pressure by applying a gate voltage.The relationships between gate voltage and transition pressure ofgraphite to diamond (temperature was maintained at 800° C.) were asfollows:

-   (1) Gate voltage (150 V): transition pressure (20 GPa)-   (2) Gate voltage (120 V): transition pressure (28 GPa)-   (3) Gate voltage (800 V): transition pressure (32 GPa)-   (4) Gate voltage (56 V): transition pressure (45 GPa)-   (5) Gate voltage (20 V): transition pressure (50 GPa)

Example 5

By doping a starting material graphite in advance with trivalent Batoms, Al atoms, Ga atoms, or In atoms, which have lower electronicnumber than carbon atoms constituting the graphite, diamond can beproduced from the graphite at lower pressure than in the case wheregraphite is singly used. In production of diamond from graphite underhigh pressure, transition from graphite to diamond was not observeduntil temperature and pressure come to high values (1500° C., 50 GPa).In contrast, a starting material graphite doped with 6% to 10% of theabove-described acceptor selected from among B atoms, Al atoms, Gaatoms, and In atoms was transformed into diamond at a low temperature(900° C.) and a low pressure (15 GPa). Conditions (temperature,pressure) under which graphite was transformed into diamond was asfollows:

-   (1) Graphite alone: high temperature (1500° C.) and high pressure    (52 GPa)-   (2) Graphite+acceptor B atoms (6%): high temperature (1400° C.) and    high pressure (45 GPa)-   (3) Graphite+acceptor Al atoms (8%): high temperature (1200° C.) and    high pressure (30 GPa)-   (4) Graphite+acceptor Ga atoms (9%): high temperature (1100° C.) and    high pressure (25 GPa)-   (5) Graphite+acceptor In atoms (10%): high temperature (900° C.) and    high pressure (15 GPa)

INDUSTRIAL APPLICABILITY

Since diamond singe-crystal thin film is formed on a semiconductor byinner-shell 1s electron excitation under normal pressure using agraphite thin film formed on the semiconductor substrate as a startingmaterial. Since the resulting diamond thin film has no dislocation ordefect, it can be used as semiconductor material in devices.

Diamond has a wide band gap, and is accordingly useful as light-emittingsources of semiconductor UV lasers and semiconductor UV light. Thepresent invention provides a material of future optoelectronics forhigh-density information. In addition, since diamond devices operate athigh temperatures, it can be applied to high-temperature semiconductordevices.

For industrial application of diamond single-crystal thin films,low-resistance p-type and n-type crystals can be formed by doping withan acceptor and a donor, and they can be used in semiconductor devicesfor high-temperature, high power electronics and UV semiconductor laserdiodes necessary for high-density recording and heavy informationcommunication.

Also, since a transparent single-crystal protective film utilizing highhardness of diamond is inexpensively and easily produced in largeamount, it is expected to be used in the field of packaging, protectivefilms, machine tools, and so forth.

The present invention produces nanostructural diamond; p-type, n-type,and nanostructural diamond doped with a dopant; and nanostructuraldiamond doped with a transition metal or rare-earth metal, fromsingle-crystal or polycrystalline graphite by inner-shell electronexcitation. These types of diamond can be industrially used as materialsfor high-power optical devices and semiconductor spin electronics.

Furthermore, the nanostructural diamond exhibits nanoscale excitonconfinement, and the confinement effect leads to strong light emissionand increase spin-orbit interaction to produce a large magnetoopticeffect. The nanostructural diamond can also be industrially used inhigh-power diamond lasers using an exciton luminescence mechanism and CWlasers using inner-shell transition of transition metal atoms orrare-earth metal atoms, emitting less temperature-dependant, sharplight.

1. A method for producing diamond, comprising the step of: exposingsingle-crystal or polycrystalline graphite having an sp² structure toone selected from the group consisting of synchrotron radiation X-rays,radiation, laser light, an electron beam, and accelerated multichargedions under normal pressure to excite the 1s inner-shell electrons ofcarbon atoms (C) constituting the graphite, thereby producing diamondhaving an sp³ structure from the graphite having the sp² structure. 2.The method for producing diamond according to claim 1, wherein exposuretime is controlled so as to produce single-crystal diamond,polycrystalline diamond, or nanostructural diamond constituted ofnanoscale particles according to the controlled exposure time.
 3. Themethod for producing diamond according to claim 1, wherein exposuretime, exposure intensity, or temperature of a substrate on which thegraphite lies is controlled so as to produce microcrystalline diamond ornanostructural diamond according to the controlled conditions.
 4. Themethod for producing diamond according to any one of claims 1 to 3,wherein the graphite used as a starting material contains at least onetype of dopant atoms selected from the group consisting of transitionmetal dopant, rare-earth metal dopant, donor dopant, and acceptor dopantand thus the diamond contains the dopant atoms.
 5. The method forproducing diamond according to any one of claims 1 to 3, wherein thegraphite having the sp² structure is provided with a gate, a source, anda drain with a metal to prepare a field-effect transistor (FET), and anegative gate voltage is applied to the FET to dope the crystal of thegraphite with a hole, whereby the diamond having the sp³ structurebecomes more stable than the graphite having the sp² structure.
 6. Themethod for procuring diamond according to any one of claims 1 to 3,wherein the graphite is doped with an acceptor, such as B atoms, Alatoms, Ga atoms, or In atoms, to allow the diamond having the sp³structure to be more stable than the graphite having the sp² structure.